Sanders, who is riding a burst of enthusiasm in Iowa, reiterated his calls for free tuition at public colleges and universities and implementing a single payer health care system that would cover all Americans, even though he'd raise taxes to pay for the latter proposal.

"Yes, we will raise taxes," said Sanders, an admission rarely heard in presidential campaigns. "We may raise taxes, but we are going to eliminate private health insurance premiums for individuals and businesses."

Sanders and Clinton spoke separately at a CNN town hall forum, fielding questions predominantly from voters still undecided ahead of the Feb. 1 Iowa caucuses, which kick off the nominating process.

Clinton pushed back at suggestions that she's new to the economic issues that have been at the center of Sanders' campaign.

"I think it's fair to say I have a 40-year record in going after inequality," said Clinton, adding that she's also fought inequality on the basis of race, gender and sexual orientation. While the questions she faced where less specific on policy, she emphasized that the tough challenges a president faces — an implicit suggestion that Sanders is proposing unrealistic ideas.

Clinton also embraced favorable comments from President Barack Obama published in Politico Monday which looked very much like an endorsement, though Obama has said he won't formally back a candidate in the Democratic primary.

"It's not only shameful and contrary to our values to say that people of a certain religion...

Former secretary of state Hillary Clinton attacked Republican front-runner Donald Trump - without ever saying his name - for denigrating Muslims with a call to bar Muslim foreigners from entering the country.

"It's not only shameful and contrary to our values to say that people of a certain religion...

(John Wagner, Abby Phillip, David A. Fahrenthold)

"I was really touched and gratified when I saw that," said Clinton, who has touted her close ties with Obama on the campaign trail and cast herself as best positioned to build on the president's policies.

While Clinton has led the Democratic field for months, she's being challenged anew by Sanders in Iowa, as well as in New Hampshire, which votes second in the primary contest.

The latest Des Moines Register/Bloomberg Politics poll found Clinton with 42 percent, Sanders with 40 percent and former Maryland Gov. Martin O'Malley with just 4 percent of likely Democratic caucus-goers. The poll, conducted between Jan. 7 and 10, had a margin of error of plus or minus 4.4 percentage points, suggesting it could be a toss-up between the former secretary of state and the Vermont senator.

Sanders, an independent from Vermont, who caucuses with Democrats on Capitol Hill, has energized young voters in particular with his call for a "political revolution."

"We are touching a nerve with the American people who understand that establishment politics just aren't bold enough," Sanders said Monday.

A self-described democratic socialist, Sanders cast his governing philosophy as one reflecting that "the right for economic security should exist." But he sidestepped a question about whether his policies would mean an era of big government.

The 74-year-old Sanders pledged to release his medical records before Iowa votes, saying there's nothing in the papers that will surprise anybody. And while Sanders typically avoids veering into personal topics, preferring to focus squarely on policy, he spoke emotionally about what his late parents would think to see their son running for president.

"This would be so unimaginable," Sanders said.

Clinton was immediately put on the defensive by a young voter who said many of his peers view her as dishonest. She vigorously disputed that notion, suggesting it was the result of decades of attacks from her political opponents.

"They throw all this stuff at me and I'm still standing," Clinton said. When asked later if she was slow to apologize for controversial use of private email and a personal Internet server while serving as secretary of state, Clinton said, "I think that's a fair criticism."

O'Malley has struggled to win support in the race, despite aggressive campaigning in Iowa. He was cheered enthusiastically when he cited climate change as the top issue young people in America should be concerned about.

O'Malley was pushed on what his supporters should do on caucus night if — under the quirks of the Iowa process — they don't reach a minimum level of support in their local precinct. Should that happen, the O'Malley backers would have to pick another candidate.

But O'Malley said his message was simply: "Hold strong at your caucus."